Soy sauce, one of typical Japanese condiments, has unique flavor, and thus, it has achieved a new recognition that it is able to exhibit an excellent cooking effect upon Western-style cuisine as well as upon Japanese cuisine. Based on such recognition, soy sauce has been increasingly demanded as a universal condiment in the United States, Europe, Southeast Asia, etc.
In general, soy sauce is produced by inoculating filamentous fungi belonging to genus Aspergillus or the like into a mixture consisting of a protein raw material such as soybeans denatured by heating and a starch raw material such as wheat, culturing the obtained mixture to produce soy sauce koji, adding the koji into saline solution, and then fermenting and aging the mixture, followed by filtration of the resultant.
A main purpose for adding the soy sauce koji into saline solution and then fermenting and aging the obtained mixture is prevention of mash putrefaction during the fermentation and aging process. The concentration of salt in saline solution, into which the koji is added, is generally between 15% and 25% by weight. Using saline solution having such a salt concentration, a soy sauce product having a common salt concentration of 10% to 20% by weight can be obtained.
On the other hand, it is said that excessive ingestion of common salts affects renal disease, heart disease, or hypertension. Since soy sauce contains a large amount of salt, there is a fear that it affects such diseases.
Under the aforementioned circumstances, various types of methods for producing a low common salt soy sauce have been studied. Examples of such a production method that has been attempted may include: a method of using saline solution with a lower concentration, such as a method of using saline solution with a minimum concentration that is necessary for avoiding putrefaction or a method of using alcohol together with mother water; and a method of producing a low common salt soy sauce by eliminating common salt from a soy sauce containing 15% to 18% by weight of the common salt obtained by a common method by means of electrodialysis, a membrane treatment and so on.
Moreover, a method of substituting a portion of common salt contained in a soy sauce with potassium chloride (KCl) has also been proposed.
Examples of such a method that has been proposed may include: an ordinary soy sauce production method, which is devised such that it comprises adding koji to a KCl solution used as mother water instead of saline solution, followed by fermentation and aging, so as to produce a KCl-containing soy sauce that contains no common salts, and at the same time, adding koji to saline solution used as mother water, followed by fermentation and aging, so as to produce a normal common salt-containing soy sauce, and then mixing both types of soy sauces, thereby obtaining a low common salt soy sauce (Patent Document 1); and a method of adding soy sauce koji to a mixed solution consisting of common salt and KCl used as mother water, followed by fermentation and aging, thereby obtaining a low common salt soy sauce (Patent Document 2).
Such a method of substituting a portion of common salt contained in a soy sauce with potassium chloride (KCl) so as to produce a low common salt soy sauce includes easy industrial operations, and thus it is extremely advantageous. On the other hand, KCl has distinctive bitter taste, which brings on a fatal demerit to a soy sauce. Accordingly, this method has been disadvantageous in that the amount of common salt substituted with KCl is consequently restricted.
A method of obtaining special nutritious foods (for example, soy sauce) for patients with hypertension, which uses soybeans, soybean germs, or ground soybeans that contain γ-aminobutyric acid-enriched soybean germs, instead of soybeans (or defatted soybeans), in a common production method of soybean processed foods (for example, soy sauce), has been known. However, it has not yet been known whether or not a soy sauce used for patients with hypertension can actually be produced by this method (Patent Document 3).
According to the studies of the present inventors, if γ-aminobutyric acid is added to a soy sauce and the content thereof is increased, the effect of suppressing elevation of blood pressure can be expected. However, it is hardly to say that such effect is sufficient.
Patent Document 1: JP Patent Publication (Kokoku) No. 38-6582 B (1963)
Patent Document 2: JP Patent Publication (Kokai) No. 56-68372 A (1981)
Patent Document 3: JP Patent Publication (Kokai) No. 11-151072 A (1199)